Avalanche photodiode operating in geiger mode including a structure for electro-optical confinement for crosstalk reduction, and array of photodiodes

ABSTRACT

An avalanche photodiode includes a cathode region and an anode region. A lateral insulating region including a barrier region and an insulating region surrounds the anode region. The cathode region forms a planar optical guide within a core of the cathode region, the guide being configured to guide photons generated during avalanche. The barrier region has a thickness extending through the planar optical guide to surround the core and prevent propagation of the photons beyond the barrier region. The core forms an electrical-confinement region for minority carriers generated within the core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application for patent Ser.No. 14/270,760, filed May 6, 2014, which claims priority from ItalianApplication for Patent No. TO2013A000398 filed May 16, 2013, thedisclosures of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to an avalanche photodiode operating inGeiger mode, which includes a structure for electro-optical confinementdesigned to limit crosstalk. Moreover, the present invention regards anarray of photodiodes.

BACKGROUND

In the technical field of photon detection, the so-called avalanchephotodiodes operating in Geiger mode (GMAPs) are known, which enabledetection of individual photons.

An avalanche photodiode operating in Geiger mode, also known assingle-photon avalanche diode (SPAD), is formed by an avalanchephotodiode APD. Hence, it comprises a junction made of semiconductormaterial, which has a breakdown voltage V_(B) and is biased, in use,with reverse-biasing voltage V_(A) higher in modulus than the breakdownvoltage V_(B), which, as is known, depends upon the semiconductormaterial and upon the doping level of the least doped region of thejunction itself. In this way, the junction has a particularly extensivedepleted region, present inside which is a non-negligible electricalfield. Hence, generation of a single electron-hole pair, caused byabsorption within the depleted region of a photon impinging on the SPAD,may be sufficient to trigger an ionization process. This ionizationprocess in turn causes an avalanche multiplication of the carriers, withgains of around 10⁶, and consequent generation in a short time (a fewhundreds of picoseconds) of the so-called avalanche current, or moreprecisely of a pulse of the avalanche current.

The avalanche current can be collected, typically by means of anexternal circuitry connected to the junction, for example by means ofappropriate anode and cathode electrodes, and represents an outputsignal of the SPAD, which will be also referred to as “output current”.In practice, for each photon absorbed, a pulse of the output current ofthe SPAD is generated.

The fact that the reverse-biasing voltage V_(A) is appreciably higherthan the breakdown voltage V_(B) causes the avalanche-ionizationprocess, once triggered, to be self-sustaining. Consequently, oncetriggered, the SPAD is no longer able to detect photons, with theconsequence that, in the absence of appropriate remedies, the SPADmanages to detect arrival of a first photon, but not arrival ofsubsequent photons.

To be able to detect also the subsequent photons, it is necessary toquench the avalanche current generated within the SPAD, arresting theavalanche-ionization process, and in particular lowering, for a periodof time known as “hold-off time”, the effective voltage V_(e) across thejunction so as to inhibit the ionization process. For this purpose, useof so-called quenching circuits, whether of an active or passive type,is known. Next, the reverse-biasing voltage V_(A) is restored in orderto enable detection of a subsequent photon.

This being said, the timing of response of the SPAD, i.e., the timerequired for generating an output current pulse following uponabsorption of a photon, is affected principally by four factors: thetime for collecting the carriers within the depleted region, typicallyof the order of a few picoseconds per micron of depleted region; thetime of propagation of the avalanche, i.e., the time required for theentire junction to be brought into the breakdown region, typically ofthe order of a few tens of picoseconds; the time of diffusion of thecarriers generated in the non-depleted region of the junction throughthe non-depleted region itself, typically comprised between a few tensof picoseconds and a few nanoseconds; and the drift time proper, forcollection of the carriers at the electrodes.

In connection with the time of diffusion of the carriers through thenon-depleted region, it should be noted that not only both of thecarriers of each electron-hole pair generated following upon absorptionof a photon within the depleted region concur to the generation of theoutput current. In fact, given the reverse biasing of the junction, tothe generation of the output current there concur also the minoritycarriers of the electron-hole pairs generated following upon absorptionof a photon outside the depleted region, hence in a non-depleted, i.e.,quasi neutral, region.

For example, assuming a junction of a PN type with the P regionarranged, with respect to the direction of propagation of the photons,upstream of the N region, there may contribute to the output currentboth the electrons of the electron-hole pairs generated in thequasi-neutral portion of the P region of the junction (also known as“dead layer”) and the holes of the electron-hole pairs generated in thequasi-neutral portion of the N region of the junction (generally knownas “epilayer”). The set of the portions of the SPAD in which generationof carriers can take place following upon absorption of photons is knownin general as “active area”.

In practice, in the present description the term “minority carriers” isused to indicate carriers that are minority carriers in the point inwhich they are generated following upon absorption of a photon. Forexample, assuming again a region of a P type, an electron of anelectron-hole pair generated following upon absorption of a photon inthis P region is a minority carrier, whereas the corresponding hole is amajority carrier.

Likewise, assuming a region of an N type, a hole of an electron-holepair generated following upon absorption of a photon in this N region isa minority carrier, whereas the corresponding electron is a majoritycarrier.

This said, the aforementioned minority carriers can cause generation ofcorresponding output current pulses, in the case where they manage todiffuse until they reach the depleted region, without first recombining.

However, even though also the minority carriers of the electron-holepairs generated outside the depleted region can contribute to photondetection, they require, in order to be able to reach the depletedregion, diffusion times that can range (according to the point ofgeneration and the doping level) between a few tens of picoseconds and afew nanoseconds. Consequently the carriers generated in the avalancheevents triggered by them can be collected at the anode and cathodeelectrodes with considerable delays. As a result, there is adeterioration of the response time of the SPAD. In particular, theso-called diffusion tails are generated in the output current.

Similar considerations may be made regarding a so-called SPAD array, andin particular, a so-called silicon photomultiplier (SiPM).

In detail, a SiPM is formed by an array of SPADs grown on one and thesame substrate and provided with respective quenching resistors (forexample, of a vertical type) integrated in the SPADs themselves, thesequenching resistors being uncoupled from one another and independent.Moreover, the anode and cathode electrodes of all the SPADs areconfigured so that they can be connected to a single voltage generator.In other words, the anode electrodes of all the SPADs are multiplexedwith one another; likewise, the cathode electrodes of all the SPADs aremultiplexed with one another. Consequently, the SPADs of the SiPM can bebiased at one and the same reverse-biasing voltage V_(A); moreover, theavalanche currents generated within them are multiplexed together so asto generate an output signal of the SiPM equal to the summation of theoutput signals of the SPADs.

In practice, the SiPM is a device having a wide area and a high gain,capable of supplying, on average, an electrical output signal (current)proportional to the number of photons that impinge upon the SiPM.However, SiPMs present the same drawbacks as the SPADs that form them.

Moreover, the SiPM, as on the other hand also a generic array of SPADsgrown on one and the same substrate, and the anode and cathodeelectrodes of which are not multiplexed with one another, is affected bycrosstalk.

In detail, given any SPAD, the corresponding operation is inevitablyaffected by charge carriers generated in surrounding SPADs and byphotons generated by electroluminescence during processes of avalanchemultiplication triggered in surrounding SPADs.

In greater detail, it may be noted that SPADs operating above thebreakdown voltage emit secondary photons as a result ofelectroluminescence, on account of various mechanisms, such as forexample intraband recombination. The secondary photons are emittedgenerally within a range of wavelengths comprised between 400 nm and 2μm, with a probability of emission that depends upon the reverse-biasingvoltage V_(A) applied.

The secondary photons can propagate and subsequently be absorbed in thedepleted regions of the junctions of SPADs different from the SPADs inwhich they have been generated, triggering avalanche events, in whichcase they give rise to the so-called “prompt crosstalk” phenomenon,which manifests itself on time scales of the order of a few tens ofpicoseconds.

It is likewise possible for the secondary photons to be absorbed withinquasi-neutral regions of SPADs different from the SPADs in which theyhave been generated, triggering avalanche events in these photodiodes,in which case they give rise to the so-called “delayed crosstalk”phenomenon, which manifests itself on time scales of the order of a fewnanoseconds. The different time scale with respect to prompt crosstalkis explained by the fact that, given a secondary photon emitted in afirst SPAD, the absorption of this secondary photon within thequasi-neutral region of a second SPAD leads to generation of a pair ofcarriers, one of which can effectively trigger an avalanche process inthe second SPAD, but only after reaching the depleted region of thesecond SPAD.

In particular, the secondary photons that most concur to the phenomenonof delayed crosstalk are the ones with wavelengths comprised between 700nm and 1100 nm, because, in this range of wavelengths, the coefficientof absorption of silicon is particularly low, and consequently thesesecondary photons can cover long distances before being absorbed.

To the phenomenon of delayed crosstalk there also concur the chargecarriers that, after being generated within the quasi-neutral regions ofSPADs, diffuse until they reach the depleted regions of SPADs differentfrom the SPADs in which they were generated. On the other hand, thesecarriers can likewise reach the depleted regions of the same SPADs inwhich they have been generated, in which case they give rise to theso-called “afterpulsing” phenomenon.

In practice, the crosstalk causes electro-optical coupling between theSPADs of the SiPMs. Consequently, the crosstalk increases the totalnoise of the SiPM, especially in the case where the SiPM has largedimensions and is subject to a high reverse-biasing voltage V_(A). As aresult, the sensitivity of the SiPM is limited; moreover, theprobability of saturating the SiPM increases, since a certain number ofSPADs is turned on owing to crosstalk, even in the absence of anexternal luminous flux.

In order to limit the crosstalk, and in particular in order to limitprompt crosstalk, the technique is known of forming, within each SPAD, atrench filled with metal material, which delimits the active area, asdescribed for example in U.S. Patent Application Publication No.2009/0184384, the disclosure of which is incorporated by reference.

As described in the document European Patent No. 1755171, the disclosureof which is incorporated by reference, likewise is known the techniqueof forming grooves of a substantially triangular shape around the activearea of each SPAD, these grooves being coated with metal material. Givena SPAD, the grooves that surround it absorb possible secondary photonsemitted by the given SPAD in directions roughly parallel to the surfaceof the given SPAD exposed to the luminous flux. In this way, a reductionof prompt crosstalk is obtained.

Moreover, given a SPAD, and considering the quasi-neutral region of thelower portion of the junction of this SPAD, the technique is known offorming an additional junction within the SPAD, arranged underneath themain junction and reverse biased in order to prevent the minoritycarriers that are generated in this quasi-neutral region from reachingthe junctions of surrounding SPADs, with consequent reduction of delayedcrosstalk. An example of this technique is described in U.S. PatentApplication Publication No. 2011/0241149, the disclosure of which isincorporated by reference. This technique thus envisages providing, foreach SPAD, three electrical terminals in order to bias the main junctionand the underlying additional junction correctly. This leads to anincrease in the complexity and a reduction in the so-called fill factorof the SiPM since part of the surface exposed to the luminous flux iscoated with two metal layers.

As regards in particular the reduction of the delayed crosstalk,likewise known are photomultipliers of the type described in thedocument PCT Application No. WO2011/132025, the disclosure of which isincorporated by reference. In detail, according to PCT Application No.WO2011/132025, each photodiode comprises a layer of amorphous silicon,which is arranged underneath the respective junction and has beencreated by means of ion bombardment of epitaxial silicon. In this way,each photodiode comprises a region with high efficiency of absorption ofphotons having wavelengths in the infrared. However, the creation of theamorphous layer by means of ion bombardment may cause an increase in thedefectiveness within the active area.

In addition, as described in PCT Application No. WO2012/083983, thedisclosure of which is incorporated by reference, techniques oftreatment of the substrate are known that enable a reduction of thenumber of photons that are reflected on the bottom surface of each SPAD,with consequent reduction of delayed crosstalk. Also in this case,however, the active area can present a certain defectiveness. Moreover,the contribution to the delayed crosstalk due to carriers generated byabsorption of the photon, before the latter reach the bottom surfaces ofthe SPADs, is not reduced.

There is a need in the art to provide an avalanche photodiode operatingin Geiger mode that will enable the drawbacks of the known art to be atleast partially overcome.

SUMMARY

According to an embodiment, an avalanche photodiode comprises: a body ofsemiconductor material having a first surface and a second surface, saidbody including: a cathode region of a first type of conductivity,forming the first and second surfaces; and an anode region of a secondtype of conductivity, extending within the cathode region and contactingthe cathode region along an interface. The photodiode further includes alateral insulating region extending through the body starting from thefirst surface and surrounding the anode region and at least part of thecathode region. The lateral insulating region comprises: a barrierregion; and an insulating region which surrounds the barrier region. Thecathode region forms an optical guide of a planar type comprising a coreregion which is arranged between the interface and the second surfaceand which extends between a minimum depth and a maximum depth and isdesigned to guide photons. The barrier region extends with a thicknessat least equal to said maximum depth so as to surround the core regionlaterally, and is configured to prevent propagation beyond the barrierregion of at least part of the photons coming, in use, from the coreregion and impinging upon the barrier region. The core region forms anelectrical-confinement region for minority carriers generated within thecore region.

In an embodiment, an apparatus comprises an array of photodiodes of thetype described above.

In an embodiment, an integrated circuit comprises: a cathode region of afirst type of conductivity; an anode region of a second type ofconductivity in contact with the cathode region; said cathode and anodeforming an avalanche photodiode; wherein the cathode region includes,underneath the anode region, a planar optical waveguide configured toguide photons generated in response actuation of the avalanchephotodiode; and an insulating region formed in the cathode regionsurrounding the anode region and passing through the planar opticalwaveguide, said insulating region configured to prevent propagation ofsaid photons beyond said insulating region.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, embodiments thereof are nowdescribed, purely by way of non-limiting example and with reference tothe attached drawings, wherein:

FIGS. 1 and 4 show cross sections of embodiments of the presentphotodiode;

FIG. 2 is a schematic view of an array of photodiodes during use;

FIGS. 3 and 5 are schematic illustrations of band diagrams for portionsof the embodiments illustrated in FIGS. 1 and 4, respectively; and

FIG. 6 shows a block diagram of a system that uses the array ofphotodiodes illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a photodiode 1 of the avalanche typeoperating in Geiger mode. The photodiode 1 may belong, for example, toan array 220 of photodiodes 1, as illustrated in FIG. 2, where also alight source 200 is represented. The array 220 may comprise any numberof photodiodes 1, according to the need.

To return to FIG. 1, the photodiode 1 is integrated in a die 100 thatincludes a substrate 2 of semiconductor material, of an N++ type andhaving a bottom surface S_(inf). Moreover, the photodiode 1 includes afirst epitaxial layer 4, a second epitaxial layer 6, and a thirdepitaxial layer 8. In FIG. 1, as on the other hand also in thesubsequent FIG. 3, the thicknesses of the substrate 2 and of the first,second, and third epitaxial layers 4, 6, 8 are not in scale.

The substrate 2 is of an N++ type, has a thickness comprised, forexample, between 300 μm and 500 μm, and has a doping level comprised,for example, between 1×10¹⁹ cm⁻³ and 5×10²⁰ cm⁻³.

The first epitaxial layer 4 is of an N−− type, has a thicknesscomprised, for example, between 5 μm and 10 μm and overlies thesubstrate 2, in direct contact therewith. Moreover, the first epitaxiallayer 4 has a doping level comprised, for example, between 1×10¹⁴ cm⁻³and 5×10¹⁴ cm⁻³.

The second epitaxial layer 6 is of an N+ type, has a thicknesscomprised, for example, between 2 μm and 5 μm and overlies the firstepitaxial layer 4, with which it is in direct contact. Moreover, thesecond epitaxial layer 6 has a doping level comprised, for example,between 1×10¹⁸ cm⁻³ and 5×10¹⁹ cm⁻³.

The third epitaxial layer 8 is of an N− type, has a thickness comprised,for example, between 2 μm and 5 μm and overlies the second epitaxiallayer 6, in direct contact therewith. Moreover, the third epitaxiallayer 8 forms a top surface S_(sup). The doping level of the thirdepitaxial layer 8 is comprised, for example, between 5×10¹⁵ cm⁻³ and1×10¹⁶ cm⁻³.

The substrate 2 and the first, second, and third epitaxial layers 4, 6,8 form a body 10 of semiconductor material, delimited by the top surfaceS_(sup) and by the bottom surface S_(inf). In what follows it isassumed, without this implying any loss of generality, that thesemiconductor body 10 is made of silicon. In addition, the firstepitaxial layer 4 extends within the body 10 with a depth of not lessthan h_(min) and not more than h_(max), this depth being measuredstarting from the top surface S_(sup).

An anode region 12, of a P+ type and circular or polygonal (for example,quadrangular) in shape, faces the top surface S_(sup) and extends withinthe third epitaxial layer 8. In particular, the anode region 12 has athickness comprised, for example, between 0.1 μm and 0.5 μm. Inaddition, the anode region 12 has a doping level comprised, for example,between 1×10¹⁹ cm⁻³ and 1×10²⁰ cm⁻³.

An enriched region 14, of an N type, extends in the third epitaxiallayer 8, underneath, and in direct contact with, the anode region 12. Intop plan view, the enriched region 14 has a circular or polygonal (forexample, quadrangular) shape. In addition, the enriched region 14 has athickness of, for example, 1 μm and a doping level comprised, forexample, between 1×10¹⁶ cm⁻³ and 5×10¹⁶ cm⁻³.

For practical purposes, the anode region 12 and the enriched region 14form a first PN junction, which is designed to receive photons andgenerate the avalanche current, as described in detail hereinafter. Inother words, the anode region 12 and the enriched region 14 are incontact with one another along an interface surface I.

The enriched region 14 and the third epitaxial layer 8 have, instead,the purpose of confining a high electrical field in the proximity of thefirst PN junction, thus reducing the breakdown voltage V_(B) of thejunction itself.

A circular guard ring 16 of a P− type with a doping level comprisedbetween 1×10¹⁶ cm⁻³ and 3×10¹⁶ cm⁻³ extends in the third epitaxial layer8. In particular, the guard ring 16 faces the top surface S_(sup) and isarranged on the outside of the anode region 12, with which it is indirect contact. Moreover, the guard ring 16 has a thickness comprised,for example, between 1 μm and 3 μm.

The guard ring 16 forms a second PN junction with the third epitaxiallayer 8 so as to prevent edge breakdown of the anode region 12.Moreover, the guard ring 16 is in direct electrical contact with ananode metallization 18, by means of which it is possible to bias thefirst PN junction. In particular, it is possible to apply to the anodemetallization 18 a reverse-biasing voltage V_(A) higher, in modulus,than the breakdown voltage V_(B) of the first PN junction.

In practice, the types of the anode region 12 and of the guard ring 16,and hence the corresponding doping levels, are such that the depletionregion that is present astride of the interface between regions of a Ptype (anode region 12 and guard ring 16) and regions of an N type(enriched region 14 and third epitaxial layer 8) extends principally inthe guard ring 16, rather than in the anode region 12, preventing theinterface between the anode region 12 (semiconductor) and the oxidelayers present on the anode region 12 (described hereinafter) frompossibly concentrating a high electrical field, thus reducing the darkcurrent of the photodiode 1. In fact, this interface is rich inShockley-Read-Hall (SRH) centers. Consequently, it is expedient to limitthe electrical fields in its proximity to prevent an undesirableincrease of the dark current of the photodiode 1.

The photodiode 1 further comprises a lateral insulating region 24, whichis arranged on the outside of the guard ring 16 and extends, startingfrom the top surface S_(sup), within the semiconductor body 10.

The lateral insulating region 24 has, in top plan view, a circular orpolygonal shape. Moreover, the lateral insulating region 24 laterallysurrounds the active area A.

In detail, the lateral insulating region 24 comprises a channel-stopperregion 27 arranged more externally, made of dielectric material (forexample, oxide) and arranged in direct contact with the semiconductorbody 10, as well as a barrier region 28, made of polysilicon, which issurrounded by the channel-stopper region 27, with which it is in directcontact.

Moreover, the barrier region 28 is in direct contact with a dielectriclayer described in detail hereinafter and referred to as “fourthdielectric layer 40”.

In greater detail, the polysilicon that forms the barrier region 28 canhave a doping either of an N type or of a P type and has a doping levelcomprised, for example, between 1×10¹⁹ cm⁻³ and 5×10²⁰ cm⁻³. Moreover,for wavelengths comprised between 700 nm and 1100 nm, the polysilicon ofthe barrier region 28 has a coefficient of absorption of approximately5×10⁴ cm⁻¹.

This being said, the barrier region 28 extends, starting from the topsurface S_(sup), with a thickness at least equal to the aforementioneddepth h_(max). Consequently, the lateral insulating region 24, inaddition to traversing the first, second, and third epitaxial layers 4,6, 8, extends at least in part in the substrate 2. The active area A ishence formed by portions of the anode region 12, of the enriched region14, and of the first, second, and third epitaxial layers 4, 6, 8.

In addition, the barrier region 28 has a width, measured along anydirection parallel to the top surface S_(sup), at least equal to 1 μm,in such a way that the barrier region 28 is able to absorb at least 99%of the photons that have wavelengths comprised between 700 nm and 1100nm and that propagate in the first epitaxial layer 4 in directionsparallel to the top surface S_(sup), until they impinge upon the barrierregion 28 itself.

Present on top of a peripheral portion of the top surface S_(sup),laterally staggered with respect to the anode region 12, is a firstdielectric layer 30, made, for example, of thermal oxide. The firstdielectric layer 30 extends partially on top of the guard ring 16.

A second dielectric layer 32, made, for example, of TEOS oxide, extendsover the first dielectric layer 30, with which it is in direct contact,as well as over the anode region 12, with which it is in direct contact.The second dielectric layer 32 extends in part also over a portion ofthe guard ring 16, with which it is in direct contact.

A coating layer 34, made, for example, of nitride, extends over thesecond dielectric layer 32 and provides, together with the latter, adouble anti-reflection coating (DLARC) 36.

By appropriately modulating, in a way in itself known, the thickness ofthe second dielectric layer 32 and of the coating layer 34, it ispossible to optimize the anti-reflection coating 36, in such a way thatit will be transparent only for a specific range of wavelengths, andreflecting for wavelengths outside said range. It is thus possible toobtain that the photodiode 1 will be sensitive only to some frequenciesof the light spectrum.

A third dielectric layer 38, made, for example, of TEOS oxide andforming a single layer with the channel-stopper region 27, extends overthe coating layer 34, with which it is in direct contact, without,however, overlying a central portion of the anode region 12. In otherwords, the third dielectric layer 38 is laterally staggered with respectto the anode region 12 and the underlying enriched region 14.

The aforementioned fourth dielectric layer 40, which is made, forexample, of TEOS oxide, extends over the third dielectric layer 38, withwhich it is in direct contact. Moreover, the fourth dielectric layer 40extends until it closes the lateral insulating region 24 at the top. Inparticular, the fourth dielectric layer 40 extends until it comes intocontact with the barrier region 28.

In practice, the anode metallization 18 traverses the second, third, andfourth dielectric layers 32, 38, 40 so as to contact the guard ring 16,as mentioned previously.

A cathode metallization 42, made of metal material, extends underneaththe bottom surface S_(inf) of the substrate 2, with which it is indirect contact. In this way, given the arrangement of the anodemetallization 18, the avalanche current flows in the direction of anaxis H, perpendicular to the bottom surface S_(inf) and to the topsurface S_(sup).

For practical purposes, the enriched region 14, the substrate 2 and thefirst, second, and third epitaxial layers 4, 6, 8 form a cathode region.Moreover, within the substrate 2, the voltage drop due to the passage ofthe avalanche current is negligible on account of the low resistivity ofthe substrate 2. Consequently, within the cathode region, the first,second, and third epitaxial layers 4, 6, 8 form a vertical integratedquenching resistor, which is electrically connected between the anoderegion 12 and the substrate 2, and is able to quench the avalanchecurrent generated following upon absorption of a photon.

As regards, instead, the lateral insulating region 24, it enables, bymeans of the barrier region 28, optical insulation of the photodiodes 1of the array 220.

In particular, the barrier region 28 of the lateral insulating region 24enables reduction of prompt crosstalk. In addition, the oxide present inthe channel stopper 27 guarantees electrical insulation between thephotodiodes 1 of the array 220, rendering the quenching resistorsindependent of one another.

It may moreover be noted that, since the lateral insulating region 24extends as far as the substrate 2, and given the low resistivity of thesubstrate 2, turning-on of a photodiode 1 does not alter, to a firstapproximation, biasing of the adjacent photodiodes 1. Consequently, thearray 220 of photodiodes 1 forms a semiconductor photomultiplier (SiPM),where the photodiodes 1 work substantially in the same operatingconditions. In this connection, even though not shown, the anode andcathode metallizations of the photodiodes 1 of the array 220 areconfigured so that they can be connected all to a single voltagegenerator, which supplies the reverse-biasing voltage V_(A).

In greater detail, thanks to the structure of the semiconductor body 10and to the characteristics of the barrier region 28, the delayedcrosstalk is particularly limited, for the reasons describedhereinafter, with reference to FIG. 3.

In particular, FIG. 3 shows the band diagram of the semiconductor body10, when the photodiode 1 is subjected to the reverse-biasing voltageV_(A), which is concentrated across the aforementioned first PNjunction. It is moreover assumed that the cathode region, i.e., thesubstrate 2 and the first, second, and third epitaxial layers 4, 6, 8,are in thermal equilibrium. It is consequently assumed that, within thecathode region, the Fermi level is constant. Moreover, for reasons ofsimplicity of presentation, it is assumed that the enriched region 14 isabsent, and hence that the anode region 12 forms the first PN junction(the depleted region of which is designated by DR) with the thirdepitaxial layer 8.

In the above conditions, within the depleted region DR of the first PNjunction, there occurs emission by electroluminescence of photons in theinfrared, in a substantially isotropic way. Referring to these photonsin the infrared as “secondary photons”, the majority of them areabsorbed within the substrate 2 and in the first epitaxial layer 4. Inany case, given the reverse biasing to which the first PN junction issubjected, the only carriers that, after being generated within thecathode region by absorption of secondary photons, can concur intriggering avalanche events are the holes.

This being said, the doping levels of the substrate 2 and of the firstand second epitaxial layers 4, 6 are such that within the firstepitaxial layer 4 a potential well is formed, i.e., a region ofelectrical confinement, for the holes. Consequently, holes generatedwithin the cathode region tend to flow towards the potential well, wherethey remain trapped and thermalize, i.e., evolve towards higher energylevels, without triggering any spurious avalanche events either inphotodiodes adjacent to the photodiode 1 or in the photodiode 1 itself.In addition, the high doping levels of the substrate 2 and of the secondepitaxial layer 6 enable reduction of the mean lifetime of the holes, onaccount of the high recombination factor, thus reducing further theprobability of triggering spurious avalanche events.

In addition, the substrate 2 and the first and second epitaxial layers4, 6 form an optical guide 50 of a planar type (“slab waveguide”),designed to confine electromagnetic radiation within the first epitaxiallayer 4. In fact, it is known that, as the doping level increases, therefractive index of silicon decreases. Consequently, the first epitaxiallayer 4 forms a sort of core with high refractive index, surrounded atthe top and at the bottom by two coatings with low refractive indices,which are formed, respectively, by the substrate 2 and by the secondepitaxial layer 6. Moreover, the core is surrounded laterally, for itsentire thickness, by the barrier region 28, from which it is physicallyseparated by just the channel stopper 27.

In practice, the optical guide 50 forms part of an optical path OP,which extends between a first portion and a second portion of thebarrier region 28 and is of a substantially guided type, but forportions that extend within the channel stopper 27. In addition, thecore of the optical guide 50, i.e., the first epitaxial layer 4, has athickness, measured parallel to the axis H, that is substantiallyconstant, in any direction parallel to the bottom surface S_(inf) and tothe top surface S_(sup).

In use, part of the photons that are generated within the firstepitaxial layer 4 and part of the photons that, albeit coming fromregions external to the first epitaxial layer 4, penetrate within thefirst epitaxial layer 4, are guided by the optical guide 50 towards thebarrier region 28, where they can be absorbed, without penetratingwithin surrounding photodiodes. In particular, the mechanism of couplingof the photons to the optical guide 50 is of a type in itself known andis such that, assuming for simplicity that the substrate 2 and thesecond epitaxial layer 6 have the same refractive index, a photonpropagates in a guided way within the optical guide 50 if it has adirection of propagation such that, in impinging on a coating of thecore, it forms an angle θ wider than the so-called critical angle θ_(c).In this case, the photon experiences the phenomenon of total reflection.

In practice, a part of the secondary photons is coupled to the opticalguide 50 and is then absorbed by the barrier region 28, without causinggeneration of carriers that concur in delayed crosstalk, with evidentbenefits in terms of reduction of the latter phenomenon.

In connection with the barrier region 28, in the embodiment illustratedin FIG. 1, as on the other hand also in the embodiments describedhereinafter, the barrier region 28, instead of being made ofpolysilicon, may be made of any material (for example, titanium nitride)that is able to absorb the secondary photons emitted byelectroluminescence, the coefficient of absorption of this absorbentmaterial and the aforementioned width of the barrier region 28 beingsuch that at least 99% of the secondary photons having wavelengthscomprised between 700 nm and 1100 nm and directions of propagationparallel to the top surface S_(sup) are absorbed.

It is likewise possible for the barrier region 28 to be made of metalmaterial, such as for example tungsten. In this case, the secondaryphotons are reflected by the barrier region 28, in a way substantiallyindependent of the width of the barrier region 28, which can hence haveany width. Also this case, the secondary photons do not penetrate withinsurrounding photodiodes, but rather remain inside the waveguide 50,until they are absorbed. The carriers thus generated in any case do notconcur in afterpulsing, for the reasons explained previously.

According to a different embodiment, illustrated in FIG. 4 and describedwith reference just to the differences with respect to the embodimentillustrated in FIG. 1, the semiconductor body 10 comprises a fourthepitaxial layer 52 and a fifth epitaxial layer 54, each of which is ofan N type, has a thickness comprised between 2 μm and 5 μm and has adoping level comprised, for example, between 1·10¹⁵ cm⁻³ and 1·10¹⁶cm⁻³.

In greater detail, the fourth epitaxial layer 52 is arranged between thefirst and second epitaxial layers 4, 6. The fifth epitaxial layer 54 isarranged between the substrate 2 and the first epitaxial layer 4.

In practice, as illustrated in FIG. 5, where for simplicity ofrepresentation it is assumed that the enriched region 14 is absent, theband diagram of the semiconductor body 10 has a stepwise profile suchthat the first epitaxial layer 4 and the fourth and fifth epitaxiallayers 52, 54 form a potential well for the holes. A region of largethickness is thus formed, which is able to facilitate thermalization ofthe holes, as well as confinement of the photons. Moreover, the fifthepitaxial layer 54 functions as buffer layer for matching the highdoping level of the substrate 2 to the low doping level of the firstepitaxial layer 4. Likewise, the fourth epitaxial layer 52 functions asbuffer layer for matching the doping levels between the first and secondepitaxial layers 4, 6.

According to further variants (not illustrated), further epitaxiallayers may be present, in such a way that arranged between the substrate2 and the first epitaxial layer 4 is a plurality of lower intermediatelayers, having doping levels decreasing progressively in the directionof the first epitaxial layer 4. In this case, present between the firstand second epitaxial layers 4, 6 is a plurality of upper intermediatelayers, having doping levels progressively increasing in the directionof the second epitaxial layer 6. Each upper intermediate layercorresponds to a lower intermediate layer in such a way as to form acorresponding pair. The layers of each pair have, for example,thicknesses and doping levels that are substantially the same as oneanother and are arranged specular to one another with respect to thefirst epitaxial layer 4.

The array 220 may be used in a generic detection system 500 illustratedin FIG. 6, where the light source 200 illuminates the array 220 and iscontrolled by a microcontroller unit 520, which is moreover connected tothe array 220. The microcontroller unit 520 biases the photodiodes 1 ofthe array 220, processes the output signal of the array 220, andsupplies a processed signal to a processor 530, which enables analysisof this processed signal and display of the information associated tosaid processed signal on a display 540.

The advantages that the present photodiode affords emerge clearly fromthe foregoing discussion.

In particular, the present photodiode has a structure that can becorrectly biased using just two terminals and enables reduction of thephenomenon of delayed crosstalk, thanks to the presence of a structurefor electro-optical confinement formed within the semiconductor body.The electro-optical confinement structure is in fact able to prevent theholes generated therein and the holes that, even though they have notbeen generated therein, arrive therein, from reaching the depletedregion of the overlying PN junction. Moreover, the electro-opticalcoupling structure forms a waveguide capable of guiding secondaryphotons in the direction of the barrier region 28, preventing the latterfrom triggering spurious avalanche events within active areas ofsurrounding photodiodes.

Finally, it is evident that modifications and variations may be made tothe photodiode described, without thereby departing from the scope ofthe present invention.

For example, the P and N types may be reversed, in which case aconfinement of the electrons, instead of a confinement of the holes, isobtained. Moreover, the anode region 12, instead of facing the topsurface S_(sup), may be overlaid by a top region of an N type. In thiscase, the anode region 12 is obtained, for example, by ion implantation,and the first PN junction is at a depth greater than the depth indicatedpreviously.

What is claimed is:
 1. An avalanche photodiode, comprising: a body ofsemiconductor material including: a cathode region of a first type ofconductivity; and an anode region of a second type of conductivity, saidanode region extending within the cathode region and contacting thecathode region along an interface; wherein the cathode region forms aplanar optical guide comprising a core region arranged below theinterface and which extends between a minimum depth and a maximum depth;a lateral insulating region that extends through the body with athickness at least equal to said maximum depth and surrounds the anoderegion and at least part of the cathode region.
 2. The photodiodeaccording to claim 1, wherein the barrier region is made of metalmaterial.
 3. The photodiode according to claim 1, wherein the barrierregion is made of optically absorbent material configured to absorbphotons propagating within the planar optical guide.
 4. The photodiodeaccording to claim 1, wherein the planar optical guide forms anelectrical-confinement region for minority carriers.
 5. The photodiodeaccording to claim 1, wherein the lateral insulating region comprises: abarrier region; and an insulating region which surrounds the barrierregion.
 6. The photodiode according to claim 1, wherein the body ofsemiconductor material is formed by a plurality of semiconductor layers,and the planar optical guide is formed by one of said plurality ofsemiconductor layers.
 7. The photodiode according to claim 6, whereinthe plurality of semiconductor layers comprises: a substrate layer; afirst epitaxial layer on the substrate layer; and a second epitaxiallayer on the first epitaxial layer; said planar optical guide formed bythe first epitaxial layer.
 8. The photodiode according to claim 7,wherein the cathode region is formed within a further epitaxial layerlocated over said second epitaxial layer.
 9. The photodiode according toclaim 8, wherein at least the first, second and further epitaxial layersform a vertical integrated quench resistor and the cathode and anoderegions form a photodiode connected to the vertical integrated quenchresistor.
 10. The photodiode according to claim 6, wherein the pluralityof semiconductor layers comprises: a first epitaxial layer; and a secondepitaxial layer on the first epitaxial layer; and a third epitaxiallayer on the second epitaxial layer; said planar optical guide formed bythe second epitaxial layer.
 11. The photodiode according to claim 10,wherein the cathode region is formed within a further epitaxial layerlocated over said third epitaxial layer.
 12. The photodiode according toclaim 11, wherein at least the first, second, third and furtherepitaxial layers form a vertical integrated quench resistor and thecathode and anode regions form a photodiode connected to the verticalintegrated quench resistor.
 13. An integrated circuit, comprising: anavalanche photodiode; a planar optical waveguide located under theavalanche photodiode and configured to guide photons generated inresponse actuation of the avalanche photodiode; and an insulating regionsurrounding the avalanche photodiode and passing through the planaroptical waveguide, said insulating region configured to preventpropagation of said photons beyond said insulating region.
 14. Thecircuit of claim 13, wherein the insulating region defines a core regionwithin which minority carriers produced in response to actuation of theavalanche photodiode are electrically confined.
 15. The circuit of claim14, wherein a cathode region of the avalanche photodiode is formed of aplurality of epitaxial layers of a first type of conductivity, andwherein said planar optical waveguide is formed by one of said pluralityof epitaxial layers.
 16. The circuit of claim 15, wherein the pluralityof epitaxial layers comprise a first epitaxial layer, a second epitaxiallayer and a third epitaxial layer, wherein the second epitaxial layerpositioned between the first and third epitaxial layers forms the planaroptical waveguide, and wherein the first and third epitaxial layers aremore highly doped than the second epitaxial layer.
 17. The circuit ofclaim 15, wherein the plurality of epitaxial layers comprise a firstepitaxial layer, a second epitaxial layer and a third epitaxial layer,wherein the second epitaxial layer positioned between the first andthird epitaxial layers forms the planar optical waveguide, and whereinthe trench passes completely through the first and second epitaxiallayers but does not pass completely through the third epitaxial layer.18. The circuit of claim 15, wherein the plurality of epitaxial layerscomprise a first epitaxial layer and a second epitaxial layer over asubstrate layer, wherein the first epitaxial layer positioned betweenthe second epitaxial layer and the substrate layer forms the planaroptical waveguide, and wherein the second epitaxial layer and substratelayer are more highly doped than the first epitaxial layer.
 19. Thecircuit of claim 13, wherein the insulating region comprises: a trenchsurrounding the anode region and extending into said cathode region; aliner formed on walls of the trench; and a fill material filling thelined trench.
 20. The circuit of claim 19, wherein the liner comprises adielectric and the fill material comprises doped polysilicon.